This invention relates to oscillator control systems which perform the function of synchronizing a locally generated signal to an external input signal. In some systems, synchronization of frequency alone is sufficient while in others synchronization of both frequency and phase is required. Toward the latter function that of synchronizing phase and frequency the present invention is primarily directed.
While phase and frequency control systems vary greatly in construction, all must perform the same two basic functions in response to either of two conditions of operation. The first, called the acquisition, arises when the frequency of the input signal and the locally generated oscillator signal are different. The function of the control systems under these conditions is to compare the frequencies of the signals and produce a proportionate error voltage which is coupled to voltage controlled oscillator in such manner that the frequency of the oscillator is changed to reduce the frequency difference. Once frequency synchronization is obtained frequency acquisition is complete and the system then functions to obtain phase lock or phase acquisition, that is, provide a substantially constant predetermined phase relationship between the reference and locally generated signals.
In the simplest systems, generally referred to as the standard automatic phase control (APC), loop, the output signal of the local oscillator and the reference signal are applied to a multiplier or phase discriminator circuit which produces an output error signal indicative of the phase relationship between the two signals. This signal is coupled via a low pass filter to a frequency control point within the voltage controlled oscillator completing the loop. During acquisition a non-symmetrical "beat" signal is produced by the multiplier which when filtered by the low pass filter provides a suitable "D.C." error voltage. Once acquisition is completed the multiplier output becomes a D.C. voltage suitable for attainment of phase synchronization.
While the APC systems perform the general function of frequency and phase synchronization satisfactorily in a variety of applications there is an inherent limitation in their performance. This limitation arises because the acquisition range and closed loop bandwidth of the system are both determined almost entirely by the bandwidth of the low pass filter and the loop gain. The criteria for wide acquisition range and stable in sync phase lock are diverse and in several respects actually oppose each other. For example increased acquisition range requires greater coupling of the nonsymmetrical beat note mentioned above which is obtained by increasing filter bandwidth. However, increases of bandwidth increase the noise bandwidths of the system causing a reduction in the stability and noise immunity of the system when phase locked. In many applications a compromise between acquisition range and noise bandwidth provides reasonable performance. However, if such compromised performance is not sufficient a standard phase lock loop may not be utilized.
This performance limitation in the standard APC loop has resulted in a number of improved systems which may be generally classified as either multiple loop systems or multiple mode systems. Typical of the former is the system set forth in U.S. Pat. No. 3,808,541 Baker et al in which an otherwise standard automatic phase control system is operated in conjunction with a frequency difference discriminator having an output signal indicative of the frequency difference between the local oscillator and reference signals. A filter network combines the outputs of the automatic phase control system and the frequency discriminating system for application to the local oscillator frequency control-circuitry.
Because the frequency difference detector output signal is zero when the frequencies of the reference and oscillator signals coincide, the output of the frequency difference loop is intended to have no effect upon the system once a frequency synchronization is obtained. In normal operation, the frequency difference output signal added to the normal automatic phase control error signal will not upset phase lock performance. However, this is true only in the absence of noise or if the input noise is symmetrical. Should the noise be asymmetrical or an interfering signal be present within the pass band of the reference signal, the frequency detector output will not be zero notwithstanding identical reference and oscillator frequencies. As a result, noise and extraneous signals may cause the frequency detector to contribute upsetting signals to the phase lock loop during in sync conditions. The acquisition and phase lock systems then are collateral but not totally independent and the in sync or phase lock performance of the combined system is determined inpart by the noise bandwidth of the frequency detector portion of the system. Conversely the acquisition range of the system may not be optimized because the filter bandwidth must be reduced for noise immunity.
Two mode operational systems may be generally described as systems which operate in either of two modes each having different loop characteristics. The acquisition mode operates with a broad filter bandwidth while the phase lock mode operates with a reduced filter bandwidth. Such systems such as the well-known quadracorrelator system set forth in proceedings of the I.R.E., January 1954, Volume 42, No. 1, Page 288, "The DC Quadricorrelator: A Two-Mode Synchronization System" by Donald Richman have shown some improvement over conventional APC systems. However, two mode systems in general exhibit considerable transitory discontinuities when changing modes. The transition between modes is often abrupt and inaccurate particularly if noise is present within the signals.